Replace RabbitMQ with Redis (issue #220)

As described in #220, our queuing mechanism for asynchronously executed jobs is hard to understand and to debug, @csenger has proposed to replace RabbitMQ with [Redis](http://redis.io/) and [RQ](http://python-rq.org/).

His motivations are described in an [email to adhocracy-dev](http://lists.liqd.net/pipermail/adhocracy-dev/2012-June/000177.html) from June 2012.

The code is now rebased against the current develop branch, and split from the caching rework (replace memcached with dogpile.cache and redis - see #230).

In my opinion this is a big improvement, but I'd be glad to have some more eyes reviewing and testing this.